Method of preparing high capacity nickel electrode powder

ABSTRACT

An electrode plate is made by loading a supporting porous metallic plaque with active battery material made by: (1) hydrolyzing the reaction product of a starting material comprising an admixture of Ni oxide and effective amounts of sodium peroxide fused at temperatures between about 800°C-1150°C, the hydrolyzed solid reaction product containing electrochemically active Ni hydrated oxides and hydroxide forms, (2) if desirable, drying the product below about 65°C, and (3) adding, at some step in the method, an amount of cobalt containing additive effective to provide about 2-12 wt% total Co in the active battery material based on Ni oxide plus Co content.

BACKGROUND OF THE INVENTION

The fusion of metallic nickel with sodium dioxide was reported in 1896by W. L. Dudley in 18 J. Am. Chem Soc. 901. Dudley fused sodium dioxidein a nickel crucible with nickel metal at a cherry-red heat, about700°-800°C, for about one hour. After cooling, the contents weresubmerged in water. The formed brown crystals were then washed to removealkali. The crystals were then dried at 110°C. The crystals wereanalyzed and believed to be the dihydrate Ni₃ O₄ . 2H₂ O, with 0.7 wt%cobalt as an impurity. A cobaltocobaltic dihydrate Co₃ O₄ . 2H₂ O isalso described as obtained by exposing to moist air Co₃ O₄, prepared byheating cobalt carbonate. These materials were believed to be newcompounds but no active battery material or electrochemical use wassuggested.

Presently used methods for the preparation of nickel active batterymaterial involve chemical precipitation or electrochemical precipitationof divalent nickel (II) hydroxide, as taught for example by Feduska et.al. in U.S. Pat. No. 3,579,385 and Hardman in U.S. Pat. No. 3,600,227.Faber, in U.S. Pat. No. 3,436,267, converted directly to trivalent Ni(III) hydroxide battery material, by 100% oxidation of finely divided Ni(II) hydroxide powder in a gas stream containing ozone. He then pastedthis material into an electrode plaque.

The usual procedure in making a battery plate involves loading thedivalent nickel (II) hydroxide into a porous plaque, with oxidation ofthe material in the plaque to a form of trivalent nickel (III)hydroxide. This is accomplished by electrochemical charging anddischarging "formation" of the loaded plaque in an alkaline electrolyte,prior to introduction of the plaque into a battery.

Ozone treatment involves a complex process using expensive equipment.Electro-precipitation processes are also costly and represent adisproportionate share of the raw materials expense in iron-nickelbatteries, while chemical precipitation methods result in gelatinousmaterials which are difficult to load into a conducting matrix.

All three of these methods involve initial production of nickelhydroxide. Chemical precipitation means high cost starting materials,precipitation, filtering, washing, drying, grinding, etc., all of whichmake the cost of the final electrode powder high. Electro-precipitationand ozone treatment involve major capital expenditures for hardware inaddition to high costs for starting materials.

With the increasing importance of improved batteries as a clean powersource, especially in the transportation area, there is a need forimproved active materials, that will provide capacities closer to thetheoretical limit than heretofore possible. To make these batteriescommercially feasible, the costs of active material manufacture must bedrastically reduced. What is needed then is a method of makinginexpensive highly active materials.

SUMMARY OF THE INVENTION

We have discovered a process that will provide an improved activatedbattery material mixture, by chemically reacting NiO, which may alsohave added to it about 2-12 wt% Co based on NiO plus Co content as amaterial selected from Co, CoO, Co₂ O₃, Co₃ O₄, or their mixtures, witheffective amounts of Na₂ O₂, generally within a weight ratio of Nio:Na₂O₂ of 1:1.35 to 1:2.1. This nickel oxide-sodium peroxide mixture isreacted at temperatures between about 800°C-1150°C, for a period oftime, generally about 1/2 - 8 hours, effective to form NaNiO₂ or NaNiO₂plus NaCoO₂ melted reaction product.

The reaction product, comprising NaNiO₂, is then hydrolyzed. If thecobalt oxide or elemental cobalt additive was not added initially,before fusion, as is preferred, it will be added generally as cobalthydroxide after hydrolysis, or as a water soluble cobalt salt such ascobalt chloride or cobalt nitrate during hydrolysis, after hydrolysis orafter plaque loading.

This process will provide a final solid active battery materialcontaining over about 95 wt% solid Ni hydrated oxides and hydroxideforms and Co hydroxide forms, the remainder being interlaminar sodium.It is important that about 0.5 to 5 wt% but preferably 0.5 to 3 wt%unreacted NaNiO₂ be present after hydrolysis and drying. The unreactedNaNiO₂ is present, in the active material as interlaminar sodium in thenickel oxy-hydroxide layers and helps prevent swelling of the activematerial in the plate during the life of the battery.

This activated battery material is washed and generally dried afterwhich it can then be loaded into a supporting porous plaque to providean electrode plate, which may then be electrochemically cycled or"formed" (electrically charged and discharged in an alkalineelectrolyte) prior to use in a battery opposite a suitable negativeelectrode. The drying step is generally carried out at temperaturesbelow about 65°C, or at a suitable temperature in a high moistureatmosphere so that water present in the active material structure is noteliminated to an extent to cause the material to lose activity.

This process involves conversion of nickel oxide, or nickel oxide withadded cobalt as elemental cobalt or cobalt oxide to an active batterymaterial powder without tedious filtering or washing steps and withoutuse of expensive electrical equipment. The starting materials cost issignificantly reduced, since nickel oxide is the least expensive nickelcontaining material commercially available. Starting with nickel oxidemakes the process useful and commercially feasible, since it eliminatesa prolonged oxidation step at high temperatures which is sure to degradethe reaction container. Starting materials cost relative to thechemical, electrochemical and ozone processes is drastically reduced byat least 50%. In addition, a by-product of this process is an aqueousalkali metal hydroxide solution which may be further used as a batteryelectrolyte by suitable processing, or used as a basic material forneutralizing mine acid pools and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be made tothe preferred embodiments exemplary of the invention, shown in theaccompanying drawings in which:

FIG. 1 is a graph showing the performance of the three Example 1 nickelelectrode plates, in terms of capacity versus cycle number, in relationto the theoretical capacity value;

FIG. 2 is a graph showing the effect of NiO + CoO + Na₂ O₂ reaction timeon the performance of nickel electrode plates;

FIG. 3 is a graph showing the effect of NiO + CoO + Na₂ O₂ reactiontemperature on the performance of nickel electrode plates;

FIG. 4 is a graph showing the effect of the weight ratio NiO:Na.sub. 2O₂ on the performance of nickel electrode plates;

FIG. 5 is a graph showing the effect of the cobalt concentration on theperformance of nickel electrode plates; and,

FIG. 6 shows a preferred electrode plaque loaded with the active batterymaterial of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of a battery, utilizing the improved active material andelectrode plate of the invention, would generally comprise a pluralityof alternate positive nickel plates and negative plates such as, forexample, loaded iron active material plates. This stack up would containplate separators between the positive and negative plates, all contactedby alkaline electrolyte and housed in a case having a cover, a vent, andpositive and negative terminals.

Preferred electrode plaques, shown in FIG. 6, are made from metalfibers, preferably nickel, or metal protective coated fibers, such asnickel coated steel or iron. A very suitable material is nickel coatedsteel wool. The plaque 10, is a flexible, expansible, compacted sheet ofrelatively smooth, generally contacting, intermingled, metal fibers asshown at 11 in the body of the plaque. The plaque has, in the embodimentshown, top edge 12 coined to a high density. The coined area provides abase to which lead tab 13, which is attached to battery terminals, isspot welded. The plaque is generally between about 90 and 95% porous.This range is preferable in providing improved conductivity andelectrolyte permeability, while maintaining enough body for good plaqueloading. Activated nickel electrode material is loaded into theinterstices of the body of this fibrous plaque to provide an electrodeplate. This invention, however, is not restricted to the preferredplaque structure described herein, and the active material can be usedwith other metallic plaque structures.

The metal fibers are preferably diffusion bonded in a protectiveatmosphere at temperatures up to the sintering point of the fibers used.In diffusion bonding, the fibers must not be melted, or protuberanceswill be formed reducing active material loading (volume) within theplaque. There should only be a metallurgical bond and interdiffusion ofatoms across the fiber interface at fiber contact points 14 along thefiber lengths. Diffusion bonding provides a flexible, expansibleelectrode structure having a large pore volume into which activematerial can be pasted or otherwise impregnated. Diffusion bonding alsolowers the electrode plate resistance appreciably and thus the internalcell resistance in a finished cell.

The active material is prepared by mixing the nickel oxide with sodiumperoxide and then heating the nickel oxide (NiO) and sodium peroxide(Na₂ O₂). These materials are generally in powdered or particulate form.The starting material preferably contains between about 2-12 wt% cobalt,based on Ni plus Co, added as elemental cobalt or preferably as a cobaltoxide such as Co₂ O₃, Co₃ O₄, CoO or their mixtures. These reactants arepreferred to be of moderate to high purity. They are fused and melted,generally in a suitable high temperature resistant container, forexample a nickel crucible, in an oxidizing or inert atmosphere, in anoven maintained at a temperature of between about 800°-1150°C, for about1/2 - 8 hours. It is essential in terms of a commercial process to usethe oxidized nickel (NiO) as starting material, since otherwise longoxidation of Ni to NiO will seriously degrade and ruin the expensivereaction container.

In the reaction, the sodium peroxide decomposes to form Na₂ O whichoxidizes the NiO. We have found, unexpectedly, that a high capacity,easily pasteable active battery material is formed when the reactionproduct is then hydrolyzed, generally by immersion in water, to cause adecomposition reaction and formation of Ni hydrated oxides and Nihydroxide forms and cobalt hydroxide. The active material is generallywashed until neutral to litmus and then may be dried at a temperaturethat will not degrade activity, e.g., between about 15°C - 65°C. TheNaOH formed could be drawn off in some continuous fashion andconcentrated by evaporation, for example, into a saleable product.

A set of equations which in part describes the basic preferred fusionand hydrolysis reactions can be given as: ##EQU1##

    2NaNiO.sub.2 + NaCoO.sub.2 + 3H.sub.2 O → NI(OH).sub.2 + CoOOH + NiO.sub.2. (1-2)H.sub.2 O + 3NaOH

We found in accordance with the prior art that cobalt addition wasnecessary at some step in the method to provide an active material inthe electrode plate which would have superior electrochemicalperformance, i.e., a capacity of about 0.185 amp-hours/gram activematerial, after 25 cycles.

The cobalt, in the form of elemental Co or cobalt oxide is addedpreferably before the fusion step, but cobalt additive may be addedinstead to the paste after the hydrolysis step, generally as cobalthydroxide Co(OH)₂, prior to incorporation into the plaque. When cobaltadditive is added as elemental cobalt or as a cobalt oxide, beforefusion, the active material contains cobalt (III) hydroxide; if added ina Co hydroxide form after hydrolysis, the active material containscobalt (III) hydroxide. Cobalt hydroxide is expensive and when addedafter hydrolysis does not provide completely homogeneous mixing.

Generally the nickel hydrated oxides and hydroxide forms will be washedto remove most of the NaOH and the cobalt hydroxide may be added as anaqueous slurry; or the nickel material may be dried and the cobalthydroxide mixed with it in a suitable mill or other type mixer. Also,during or after hydrolysis, aqueous cobalt chloride (Co(Cl)₂ . (6H₂ O)or cobalt nitrate (Co(NO₃)₂ . 6H₂ O) additive may be used, in which caseafter reaction with alkaline hydroxide present or added, the finalactive material will contain cobalt (II) hydroxide, Co(OH)₂. Addition ofan appropriate amount of cobalt nitrate solution to the alkaline slurryafter hydrolysis results in a fairly uniform dispersion of Co(OH)₂precipitate with the nickel active material.

The plaque can also be loaded with battery material not containingcobalt, and then dipped for an adequate period of time in aqueous cobaltnitrate or chloride solution, dried, and finally dipped in alkalihydroxide, such as KOH, NaOH or LiOH, to provide a precipitate ofCo(OH)₂ in the material. This would also provide a useful method toupgrade the cobalt content of loaded plaques.

In all cases, cobalt addition is preferred and the total weight percentof cobalt, as Co in the active material, must be between about 2 - 12wt% and preferably about 5-8 wt% of the initial weight of NiO plus Co,i.e., wt% Co = Co/(NiO + Co) Cobalt concentration below 2 wt% and above12 wt% detracted from acceptable performance. Failure to add cobalt tothe plaque provided a plate having a capacity of about 0.10 amphr/g. Auseful active material can be made without containing cobalt, but aplate containing such material, before being used in a battery, shouldbe dipped in a cobalt solution to insure cobalt content and obtaininghigher capacity.

We found that the weight ratio of NiO to Na₂ O₂ was critical inproviding an electrode plate having acceptable electrochemicalperformance. The weight ratio of NiO:Na₂ O₂ must be between about 1:1.35to about 1:2.1. An amount of Na₂ O₂ less than about 1.35 parts per 1part NiO would provide relatively poor performance. A 1:1 weight ratioof NiO:Na₂ O₂ provided a mixture that remained in slurry form withincomplete reaction. An amount of Na₂ O₂ over about 2.1 parts per 1 partNiO causes rapid destruction of the reaction vessel and does not provideincreased electrochemical capacity. Other peroxides similar to sodiumperoxide, such as lithium peroxide or lithium oxide and potassiumsuperoxide as well as barium, strontium, and calcium alkaline earthperoxides were found unsuitable.

We found that the temperature and time of reaction melt fusion of theNiO and Na₂ O₂ influenced the capacity of the active product.Temperatures around 700°-750°C provided a reaction mixture that wasstill somewhat semi-solid, indicating slow and incomplete formation ofNaNiO₂ and incomplete intimate reaction of the NiO and Na₂ O₂. At atemperature of 600°C most of the NiO does not react. Temperatures held,after heating the oven, at over 1150°c provide materials problems infinding suitable reaction vessels which will not degrade very quicklyand add deleterious materials to the fused NaNiO₂.

The useful temperature range for complete fusionreaction, to bemaintained after heating the oven, is between about 800°-1150°C. Thepreferred fusion-reaction temperature range, to be maintained afterheating the oven, is from about 850°-1100°C. The most preferredtemperature range in order to assure reuse of the preferred nickelreaction vessel is between about 850°C -925°C. The time necessary forfusion will vary depending on temperature. At 800°-850°C, 6-8 hours isgenerally sufficient for complete reaction, while at 850°-1100°C orhigher, 1/2-3 hours is generally adequate. The best performance wasobserved at a fusionreaction temperature of 1000°C for 2 hours.

The water temperature for the hydrolysis reaction of the NaNiO₂ can bebetween about 10°C -95°C but preferably between about 20°C -35°C. Whenreacted between 20°C-35°C the reaction provides more Ni (III) hydroxidei.e., a weight ratio of Ni (II) hydroxide:Ni (III) hydroxide of overabout 1:2 providing better electrochemical properties. A higherconcentration of the more crystalline Ni (III) hydroxide also provides acomposition that loads better into the plaque. The molten NaNiO₂ can bequenched in water at NaNiO₂ temperatures below about 600°C, i.e., theNaNiO₂ can be cooled to below 600°C and then immersed in water; thishowever produces a very active hydrolysis, and it is preferred to coolthe NaNiO₂ to between 20°C -95°C before hydrolysis. Also of particularadvantage in this method, NaOH solution is produced which may be furtherused as a battery electrolyte.

The final active material will contain nickel hydrated oxides andhydroxide forms plus cobalt hydroxide. it will also contain about 0.5 to5 wt% but generally about 2 wt% unhydrolyzed or unreacted NaNiO₂, basedon dried nickel hydrated oxides and hydroxides plus cobalt hydroxideforms. This sodium material imparts important reduced swellingproperties. The active material is then washed and dried. This materialcan be made into a high density fluid active battery paste forapplication to battery plaques. The active material after drying up to65°C contains water molecules between spaced --O-Ni-O-- layers. It isessential that the water remain in the structure. Therefore, drying isof a partial nature and must be accomplished at a temperature andhumidity effective to retain an optimum amount of the interlaminar H₂ O.Generally the temperature limits are between about 15°C to 65°C with apreferred range of 20°C-40°C. Above 65°C drying and the electrochemicalactivity starts to decrease. Above 100°C drying, the electrochemicalactivity continues to decrease to the extent that the material starts tobecome inactive. Above 130°C involves complete drying and the cubic Nioelectrochemically inactive state is formed.

For simplicity, one of the nickel hydroxide forms comprising the finalhydrated active material has been written as nickel (III) hydroxide.This is a simplified way of stating an average between Ni (II) and Ni(IV) hydroxides. There is considerable speculation as to the preciseformula of the higher valent, oxidized nickel hydroxide. Analysis ofseveral samples of hydrolyzed NaNiO₂ were obtained using thedimethylglyoxime gravimetric technique. The results indicate that aprimary nickel compound corresponds to a stoichiometry of Ni₃ O₄.2H₂ O,a nickel oxide hydrate. For the purposes of this application, the termnickel (III) hydroxide and nickel hydrated oxides and hydroxide formswill be used to identify the electrochemically active nickel compoundobtained by the substantially complete chemical hydrolysis reaction ofNaNiO₂.

The sodium peroxide, nickel oxide, cobalt and cobalt oxide startingmaterials, as well as cobalt hydroxide and water soluble cobalt saltadditives are preferred to be substantially pure, i.e., no more thanabout 5% of electrochemically harmful impurities that cannot be washedaway. Fortunately, commercial grades of black nickel oxide powder aresufficiently pure to be used as supplied.

EXAMPLE 1

An electrode powder active battery material containing about 98 wt%cobalt . nickel hydroxide was mixed by placing in a container andthoroughly blending 7.6 grams (0.10 mole) of 99^(+%) pure, finelydivided black nickel oxide, NiO, (containing 78 wt% or 5.9 grams Ni) and0.38 grams (0.005 mole) of 99% pure cobalt oxide, mostly in the form ofCoO, (containing 70 wt% or about 0.27 gram Co) with 11.7 grams (0.15mole) of C. P. (96.5% chemically pure) grade sodium peroxide, Na₂ O₂.The nickel oxide consisted essentially of NiO and was commerciallyavailable as INCO black NiO; the cobalt oxide comprised mostly CoO andwas commercially available as BAKER reagent cobalt oxide, containing 70wt% Co. This provided approximately a 3.4 wt% cobalt concentration basedon nickel oxide plus cobalt content, i.e., 0.27 g Co divided by (7.6 gNiO + 0.27 g Co); and a weight ratio of NiO:Na₂ O₂ of about 1:1.54.

This mixture was then placed in a nickel crucible and gradually heatedfor about 1 hour up to about 800°C in air, in a ceramic lined oven withNichrome heating coils. Temperatures were monitored using a Pt-PtRhthermocouple introduced at the rear of the oven. After the oven washeated up to 800°C, the temperature was increased and maintained at thefusion-reaction temperature of between about 950°C- 1025°C for anadditional 1 hour, to ensure substantially complete chemical melt-fusionreaction to a substantially pure NaNiO₂ NaCoO₂ mixture.

The crucible and reaction product contents were then cooled to about25°C over a 6 hour period, after which the crucible containing a solidmass of material was immersed in a 250 ml beaker of water at about 25°C.The contents hydrolyzed over a 12-hour period, and dispersed in thewater to provide an active battery material powder containing about 98wt% reacted oxide hydrates and hydroxides with about 2 wt% sodium on adried basis as unreacted NaNiO₂. The heavy brown-black solid activematerial settled immediately in the beaker and was separated using aconventional Buchner apparatus. It was washed with successive 100 mlportions of water until neutral to litmus. This provided a densebrown-black crystalline powder material. It was noted that the nickelcrucible was somewhat degraded after the reaction. The filtrateconsisted of NaOH solution, which could be used later as a batteryelectrolyte.

The active battery powder was then air dried at only 25°C, so as not toeliminate interlaminar water in the crystals, and sieved to -325 mesh,i.e., about 98% of the powder had a diameter of less than about 44microns. This powder was then loaded into nickel battery plaques orgrids. The grids were 90-95% porous, 100 mil thick diffusion bondednickel plated steel wool fiber plaques, having an area of about 1 sq.in. They were loaded using a conventional suction platform. An aqueousslurry of the active material was made to provide a high density fluidpaste which was added from a blender until the plaques were filled.Additional water was dropped onto the loaded electrode plates from afunnel to obtain optimum packing within the plaque support.

Sample 1(a) electrodes, having an initial thickness of about 100 mils,were than pressed at about 20,000 lb/sq.in., to a final thicknessapproximating 60 mils. The loading in each plaque was about 1.5grams/sq.in. of plaque surface area. Sample 1(b) electrodes, having aninitial thickness of about 100 mils, but more heavily loaded, were thenpressed at about 20,000 lb/sq.in., to a final thickness approximately 80mils. The loading in each plaque was about 2.5 grams/sq. in.

The nickel electrodes of Samples 1(a) and 1(b) were set oppositenegative electrodes in several containers, and contacted withelectrolyte to form electrochemical cells. The nickel hydroxideelectrodes were "formed", i.e., charge and discharge cycled versussintered cadmium electrodes of considerably larger size and capacity.They were charged for about 21/4 hours at a current density of about 0.3amp/sq.in. in 25 wt% aqueous KOH and discharged through a 10 ohmresistor at a current density of approximately 120 mA/sq.cm. in 25 wt%aqueous KOH electrolyte. The amount of charge for each cell was adjustedto about 250% of the theoretical nickel capacity based on a singleelectron transfer per nickel atom.

The cycling increases the porosity of the electrode, allowing increasedelectrolyte penetration and higher output. Initially, the activematerial is tightly packed and the electrolyte is restricted fromcontacting the interior of the electrode. An electrode is ready for useafter "forming" for about 10 to 35 cycles. The active material after"formation" did not show any appreciable swelling in the batteryelectrode plate.

Capacity values which we considered acceptable for nickel hydroxidebattery material were over about 0.185 amphr/gram active material after25 cycles. This would provide an active battery material highlyeffective in approaching theoretical values and much improved over theprior art. Theoretical values for one-electron transfer, at 0.255 amphr/g, are shown on FIG. 1 as a broken horizontal line. The capacity ofthe electrodes made by the method described above are also shown on FIG.1, as curves 1(a) and 1(b), providing a capacity at 25 cycles of betweenabout 0.225- 0.20 amp-hr/g. The electrochemical performance of Sample1(b) is about the same as for Sample 1(a) even though the electrode ismuch thicker and more heavily loaded. This is of particular advantage,indicating that low loadings, providing even lower materials costs, willstill provide excellent electrochemical results.

An electrode powder active battery material, Sample 1(c), containingabout 98 wt% reacted oxide hydrates and hydroxide forms was made byadding about 10 grams reagent grade, 99% pure, cobalt nitrate solution,Co(NO₃)₂. 6H₂ O, (containing 20 wt% or 0.35 gram cobalt) to the stronglyalkaline hydrolyzed slurry after the crucible was immersed in water,rather than adding cobalt additive to the NiO + Na₂ O₂ mixture beforefusion. This provided an active battery material containing cobalt (II)hydroxide dispersed throughout. The mixture before fusion contained 7.6grams NiO and 11.7 grams C.P. grade sodium peroxide, providing a weightratio of NiO:Na₂ O₂ of about 1:1.54. The same fusion, cooling,hydrolysis and washing cycle was followed as described for Samples 1(a)and 1(b) above. The cobalt concentration based on nickel oxide plus Cocontent was 4.4 wt%, i.e., 7.6 g. Ni:0.35 g. Co. This active batterymaterial was loaded, pressed, set opposite negative electrodes to form acell, and charged and discharged as described above for Sample 1(a).

The capacity of this electrode is also shown in FIG. 1, as curve 1(c),providing a capacity at 25 cycles of about 0.19 amps-hr/g. Mixing thecobalt additive initially as CoO, as in Samples 1(a) and 1(b), appearsto provide an electrode active material with more capacity, probablybecause of the intimate interaction with NiO and Na₂ O₂ during themelt-fusion reaction, and is the preferred method of adding cobalt tothe battery material.

The cobalt could also be added by dipping or spraying a plaquecontaining the nickel hydroxide after loading, followed by dipping orspraying with high purity alkali hydroxide to precipitate cobalthydroxide and then washing the plaque. When the cobalt weight percent isbetween 2-12 wt% the results would be similar to Sample 1(c). Cobalt canalso be added as high purity Co(OH)₂ after hydrolysis and when thecobalt weight percent is between 2-12 wt% the results would again besimilar to Sample 1(c). None of the post fusion cobalt addition methods,either as a cobalt hydroxide or as soluble cobalt nitrate or cobaltchloride salts, provide as complete a homogeneous mixing of the cobaltwith the nickel hydroxides.

Nickel analysis of several batches of the hydrolyzed NaNiO₂ of Samples1(a) and 1(b) were obtained, using the dimethyglyoxime gravimetrictechnique. The results indicate 50-60 wt% nickel due to varying degreesof hydration, plus cobalt, oxygen and residual sodium, as shown in TABLE1 below:

                  TABLE 1                                                         ______________________________________                                        Nickel analysis for fully hydrolyzed NaNiO.sub.2 samples                      Sample Wt. (g.)                                                                              Ni Content (g.)  % Ni                                          ______________________________________                                        0.155          0.0890           57.6                                          0.147          0.0902           61.4                                          0.151          0.0842           55.7                                          0.148          0.0840           56.8                                          ______________________________________                                    

The leaching of sodium from NaNiO₂ was very slow. Even after a week ofwashing a sample of finely ground NaNiO₂ with water, the sodium contentwas still about 1.0 percent.

The hydrolyzed NaNiO₂ starts to lose significant weight above 130°C.Some interlaminar water is believed to be lost at heating temperaturesover about 45°C-65°C. In the range of 130°C-240°C, the compound losesoxygen and residual water amounting to about 13.0 wt%. Heating aboveabout 130°C involves complete drying and eliminates almost allinterlaminar water, causing Ni in the crystal structure to link with 4-6oxygen atoms forming a cubic electrochemically inactive state. Thevalues resulting from the dimethyglyoxime analysis of the activematerial closely correspond to a stoichiometry of nickelo-nickelichydrate, Ni₃ O₄ . 2H₂ O, in a layer like --O-Ni-O-Ni-O-Ni-O-- form withinterlaminar water, which on conversion to NiO, would lose about 18.9%of its weight. The nickel content of Ni₃ O₄ . 2H₂ O is calculated to beabout 63.6%. The values given in Table 1, when corrected for the 5-6%cobalt additive, agree with this percentage reasonably well.

EXAMPLE 2

An electrode powder active battery material was mixed by placing in acontainer and thoroughly blending 7.6 grams of 99^(+%) pure NiO(containing 5.9 grams Ni) and 0.56 grams of 99% pure cobalt oxide(containing about 0.44 grams Co) with 11.7 grams of C.P. grade sodiumperoxide, Na₂ O₂. This provided approximately a 5.5 wt% cobaltconcentration based on nickel oxide plus cobalt content, i.e., 7.6 g.NiO + 0.44 g. Co. and a weight ratio of NiO:Na₂ O₂ of about 1:1.54.

This mixture, i.e., NiO:1.5 Na₂ O₂ with 5.5 wt% Co, was then placed innickel crucibles and 4 batches, Samples 2(a) -2(d), gradually heatedabout 1 hour up to 800°C in air, in a ceramic lined oven with Nichromeheating coils. Temperatures were monitored using a Pt-PtRh thermocoupleintroduced at the rear of the oven. The temperature was then increasedand maintained at the chemical fusion-reaction temperature of 1000°C forfurther periods of 20 minutes, Sample 2(a); 1 hour, Sample 2(b); 1.5hours, Sample 2(c); and 3 hours, Sample 2(d); to determine the effect offusionreaction time on the electrochemical performance of the activebattery material.

The crucible was then cooled to 25°C and immersed in water at 25°C tohydrolyze and disperse the contents. The hydrolyzed material containingabout 98 wt% reacted hydrated oxide and hydroxide forms and 2 wt%unreacted NaNiO₂ on a dried basis, was then washed until neutral tolitmus, dried at 25°C, loaded into nickel battery grids, and setopposite negative electrodes to form cells; all steps using the sametechniques as described in Sample 1(a) in EXAMPLE 1.

The electrochemical activity of these electrodes are shown on FIG. 2.The material heated at 1, 1.5 and 3 hours at the fusion-reactiontemperature of 1000°C provided capacity values at 25 cycles of about0.21, 0.22 and 0.23 amp-hr/g. The material heated for 20 minutes at thefusionreactor temperature provided capacity values at 25 cycles of 0.17amp-hr/g. Suitable reaction times at fusion-reaction temperatures ofbetween 800°-1150°C would appear from this data to be over aboutone-half hour and probably up to about 8 hours at the 800°C temperaturerange.

EXAMPLE 3

An electrode powder active battery material was made as in Example 2,i.e., NiO:1.54 Na₂ O₂ with 5.5 wt% Co. Samples of this mixture were thenplaced in a nickel crucible and heated for 6 hours at 600°C, Sample 3(a); 6 hours at 700°C, Sample 3(b); 6 hours at 800°C, Sample 3(c); 6hours at 850°C, Sample 3(d); and 2 hours at 1100°C, Sample 3 (e), afterinitial furnace heating, to determine the effect of temperature on theelectrochemical performance of the active battery material. The furnacewas the same type used in EXAMPLE 2.

The crucible was then cooled to 25°C and immersed in water at 25°C tohydrolyze and disperse the contents. The hydrolyzed material, containingabout 98 wt% reacted hydrated oxide and hydroxide forms and 2 wt%unreacted NaNiO₂ on a dried basis, was then washed, dried at 25°C,loaded into nickel battery grids, and set opposite negative electrodesto form cells, all steps using the same techniques as described inSample 1(a) in EXAMPLE 1.

The electrochemical activity of these electrodes are shown on FIG. 3,where the materials heated at 800°, 850° and 1100°C provided capacityvalues at 25 cycles of about 0.19, 0.20 and 0.23 amp-hr/g respectively.The materials heated at 600° and 700°C provided capacity values at 10cycles of only about 0.05 and 0.15 amp-hr/g. Suitable fusion-reactiontemperatures would appear from the data to be between about 800°C to1100°C or higher, although at temperatures above about 925°C the nickelreaction vessel shows signs of deterioration.

EXAMPLE 4

Electrode powder active battery material was mixed by placing in acontainer and thoroughly blending 7.6 grams of 99^(+%) pure NiO(containing 5.9 grams Ni) and 0.56 grams of 99% pure cobalt oxide(containing 0.4 grams Co), to provide a 5.5 wt% cobalt concentration,with about 7.6 grams of C.P. grade Na₂ O₂, Sample 4(a); about 9.5 gramsof C.P. grade Na₂ O₂, Sample 4 (b); about 11.7 grams of C.P. grade Na₂O₂, Sample 4 (c); and about 15.2 grams of C.P. grade Na₂ O₂, Sample4(d). This provided weight ratios of NiO:Na₂ O₂ of about 1:1; 1:1.25;1:1.5; and 1:2.0 respectively. The mixtures were then placed in a nickelcrucible, heated, and then fuse-melted at a fusion-reaction temperatureof 1000°C for 3 hours.

These materials were then cooled to 25°C, and immersed in water at 25°Cto hydrolyze them to Co hydroxide plus Ni hydrated oxide and hydroxideforms and about 2 wt% unreacted NaNiO₂ on a dried basis. The materialwas then washed, dried at 25°C, loaded into nickel battery plaques at aloading of about 1.5 grams/sq.in., and set opposite negative electrodesto form a cell, all steps using the same techniques as described forSample 1(a) in EXAMPLE 1.

The electrochemical activity of these electrodes are shown on FIG. 4,where the materials having weight ratios of NiO:Na₂ O₂ of 1:1.5 providedcapacity values at 25 cycles of about 0.23 and 0.22 amp-hr/g. Thematerials having weight ratios of NiO:Na₂ O₂ of 1:1 and 1:1.25 providedcapacity values at 25 cycles of about 0.08 and 0.15 amp-hr/g. Suitableweight ratios of NiO:Na₂ O₂ would appear from the FIgure data to bebetween about 1:1.35 to about 1:2.1.

EXAMPLE 5

Electrode powder active battery material was mixed by placing in acontainer and thoroughly blending 7.6 grams of 99^(+%) pure NiO(containing 78 wt% or 5.9 grams Ni) and 11.7 grams of C.P. grade Na₂ O₂,to provide a weight ratio of NiO:Na₂ O₂ of 1:1.54, with about 0.23 gramsof 99% pure cobalt oxide (containing 70 wt% or 0.16 gram Co), Sample5(a); about 0.38 grams of 99% pure cobalt oxide (containing 0.3 gramCo), Sample 5(b); about 0.7 grams of 99% pure cobalt oxide (containing0.49 gram Co), Sample 5(c); about 0.9 grams of 99% pure cobalt oxide(containing 0.63 gram Co), Sample 5(d); and Sample 5(e) containing nocobalt addition either before fusion or after hydrolysis. This providedcobalt concentrations, based on nickel oxide plus cobalt content as Coof approximately about 2.1 wt%, 3.8 wt%, 6.1 wt%, 7.6 wt%, and 0 wt%respectively. The mixtures were then placed in a nickel crucible,heated, and then fuse-melted at a fusion-reaction temperature of 1000°Cfor 3 hours.

These materials were then cooled to 25°C, and immersed in water at 25°Cto hydrolyze them to about 98 wt% reacted hydrated oxide and hydroxideforms and 2 wt% unreacted NaNiO₂ on a dried basis. The materials werethen washed, dried at 25°C, and loaded into nickel battery plaques, atloadings of 1.7 grams/sq.in. for a final plaque thickness of 70 mils,and set opposite negative electrodes to form a cell, all steps using thesame techniques as described for Sample 1(a) in EXAMPLE 1.

The electrochemical activity of these electrodes are shown on FIG. 5,where the materials having cobalt concentrations of 2.1 wt%, 3.8 wt%,6.1 wt%, and 7.6 wt% provided the capacity values at 25 cycles of about0.22, 0.22, 0.24 and 0.20 amp-hr/g. respectively. The material withoutcobalt provided a capacity value of only 0.10 amp-hr/g. after 25 cycles.Suitable cobalt addition from this data to provide an acceptableelectrode plate ready for insertion into a battery would appear to bebetween about 2 wt%-12 wt% based on nickel oxide as NiO plus cobaltcontent as Co.

EXAMPLE 6

As a comparative example, a material, Sample 6, containing between about95-98 wt% hydrolyxis reaction product was mixed by placing in acontainer and thoroughly blending 30 grams of 99^(+%) pure NiO,(containing 24 grams Ni), and 2.2 grams of 99% pure cobalt oxide(containing 1.5 grams Co) with 50 grams of C.P. grade potassiumsuperoxide, KO₂. This provided a 4.8 wt% cobalt concentration based onnickel oxide plus cobalt content and a weight ratio of NiO:KO₂ of about1:1.65. This mixture was then placed in a nickel crucible and heatedover about 1 hour to 800°C. The temperature was raised and maintainedbetween 950°-1020°C for about 2 hours to ensure complete melt fusionreaction to KNiO₂ + KCoO₂.

This material was then cooled to 25°C, and immersed in water at 25°C tohydrolyze it. The material was then washed, dried at 25%, loaded intonickel battery plaques at a loading of about 1.5 grams/sq.in., and setopposite negative electrodes to form a cell, all steps using the sametechniques as described for Sample 1(a) in EXAMPLE 1. The capacity ofthese electrodes, Sample 6, made substituting KO₂ for Na₂ O₂, provided acapacity of 0.12 amp-hr/g. after about 25 cycles, indicating muchinferior electrochemical performance for this material.

EXAMPLE 7

As a comparative example, an electrode was made, similarly to Sample1(a) in EXAMPLE 1, where Li₂ O was substituted for Na₂ O₂ in themixture. The material contained about 3.4 wt% cobalt concentration and aweight ratio of NiO:Li₂ O of about 1:1.5. This mixture was fused at1000° C to ensure substantial reaction to CoS.Li₂ O, although themixture remained solid due to the high melting point of Li₂ O, cooled to25°C, and hydrolyzed in water at 25°C. The material was then washed,dried at 25°C, loaded into nickel battery grids, and set oppositenegative electrodes to form a cell, all steps using the same techniquesas described for Sample 1(a) in EXAMPLE 1. The final product was agrayblack powder with some dispersed metallic-like platelets. Theelectrodes, Sample 7(a), made by substituting Li₂ O for Na₂ O₂, had onlysuperficial electrochemical activity. X-ray and infrared reasurementsindicated that the LiNiO₂ contained a cubic NiO structure and not thelayer-like NiO₂ structure found in NaNiO₂ necessary for electrochemicalactivity.

Also, electrode Sample 7(b) was made using the same techniques asdescribed for Sample 1(a) in EXAMPLE 1, but substituting bariumperoxide, BaO₂, for Na₂ O₂. The final product, BaNi₂ O₅ + BaCo₂ O₅ hadno electrochemical activity. X-ray structural patterns also showed NiOsites. In addition, the hydrolysis step produced Ba(OH)₂ which wasinsoluble, inactive dead weight and difficult to separate from thebarium cobalt and nickel oxides by washing.

Tabulated results of the Examples are shown in the following TABLE 2:

                                      TABLE 2                                     __________________________________________________________________________    ELECTRODE MATERIAL CAPACITY: Charge at 300 mA for 2 1/4 hrs.; discharge       at 120 mA - in 25% KOH                                                                   Approximate % Co              Capacity                             Wt. Ratio  in Final Hydro-                                                                          Fusion Temp.                                                                            Plaque Loading                                                                         amp-hr/gram                          Sample                                                                            NiO:Na.sub.2 O.sub.2                                                                 lyzed Material                                                                            and Time g/sq.in. 10 cycles                                                                            25 cycles                     __________________________________________________________________________    1(a)                                                                              1:1.54 3.4%      950°-1025°C (1 hr)                                                         1.5       0.215  0.225                        1(b)                                                                              1:1.54 3.4%      950°-1025°C (1 hr)                                                         2.5      0.20   0.20                          1(c)                                                                              1:1.54 4.4%      950°-1025°C (1 hr)                                                         1.5       0.185 0.19                          2(a)                                                                              1:1.54 5.5%      1000°C (20 min)                                                                   1.5      0.17   0.17                          2(b)                                                                              1:1.54 5.5%      1000°C (1 hr)                                                                     1.5      0.21   0.21                          2(c)                                                                              1:1.54 5.5%      1000°C (1.5 hr)                                                                   1.5      1.22   0.22                          2(d)                                                                              1:1.54 5.5%      1000°C (3 hr)                                                                     1.5      0.23   0.23                          3(a)                                                                              1:1.54 5.5%      600°C (6 hr)                                                                      1.5      0.05   none                          3(b)                                                                              1:1.54 5.5%      700°C (6 hr)                                                                      1.5      0.15   0.15                          3(c)                                                                              1:1.54 5.5%      800°C (6 hr)                                                                      1.5      0.17   0.19                          3(d)                                                                              1:1.54 5.5%      850°C (6 hr)                                                                      1.5      0.19   0.20                          3(e)                                                                              1:1.54 5.5%      1100°C (2 hr)                                                                     1.5      0.23   0.23                          4(a)                                                                              1:1.0  5.5%      1000°C (3 hr)                                                                     1.5      0.08   0.08                          4(b)                                                                              1:1.25 5.5%      1000°C (3 hr)                                                                     1.5      0.16   0.16                          4(c)                                                                              1:1.5  5.5%      1000°C (3 hr)                                                                     1.5      0.23   0.23                          4(d)                                                                              1:2.0  5.5%      1000°C (3 hr)                                                                     1.5      0.22   0.22                          5(a)                                                                              1:1.54 2.1%      1000°C (3 hr)                                                                     1.7      0.22   0.22                          5(b)                                                                              1:1.54 3.8%      1000°C (3 hr)                                                                     1.7      0.22   0.22                          5(c)                                                                              1:1.54 6.1%      1000°C (3 hr)                                                                     1.7      0.24   0.24                          5(d)                                                                              1:1.54 7.6%      1000°C (3 hr)                                                                     1.7      0.20   0.20                          5(e)                                                                              1:1.54 0%        1000°C (3 hr)                                                                     1.7      0.10   0.10                          6   NiO:KO.sub.2                                                                         4.8%      950°-1025°C (2 hr)                                                         1.5      --     0.12                              1:1.65                                                                    7(a)                                                                              NiO:LiO.sub.2                                                                        3.4%      950°-1025°C (1 hr)                                                         1.5      none   none                              1:1.54                                                                    7(b)                                                                              NiO:BaO.sub.2                                                                        3.4%      950°-1025°C (1 hr)                                                         1.5      none   none                              1:1.54                                                                    __________________________________________________________________________

EXAMPLE 8

In order to determine the effect of drying temperature on theelectrochemical capacity of the active material, an electrode was made,similarly to Sample 2(d) in EXAMPLE 2 but using drying temperaturesafter hydrolysis of 25°C, 30°C, 40°C, 45°C, 70°C and 90°C. The materialcontained about 6.5 wt% cobalt concentration and a weight ratio ofNiO:Na₂ O₂ of about 1:1.54. The mixture was fused in a nickel crucibleat 1000°C as in EXAMPLE 2 to ensure complete conversion to NaNiO₂. Aftercooling and hydrolysis, the active battery material powder containedabout 98 wt% reacted hydrated oxide and hydroxide forms and 2 wt%unreacted NaNiO₂ on a dried basis.

The material was washed and then Samples 8(a)-8(f) were air dried at theabove described temperatures. The Samples were then sieved to -325 meshand loaded into nickel battery plaques as in EXAMPLE 2. The electrodeswere pressed and had approximately the same loadings as in EXAMPLE 2.These nickel electrodes were set opposite negative electrodes in severalcontainers, and contacted with electrolyte to form electrochemicalcells. The electrodes were fromed as in EXAMPLE 2. The capacity of theelectrodes after level performance was attained at about 25 cycles isshown in the following TABLE 3:

                                      TABLE 3                                     __________________________________________________________________________    ELECTRODE MATERIAL CAPACITY: Charge at 300 mA for 2 1/4 hrs.;                 discharge at 120 mA - in 25% KOH                                                                                   Capacity                                             Wt. Ratio                                                                            Fusion Temp.                                                                           Plaque Loading                                                                         amp-hr/gram                              Drying Temp.                                                                          Sample                                                                            NiO:Na.sub.2 O.sub.2                                                                 and Time g./sq.in.                                                                              25 Cycles                                __________________________________________________________________________    25°C                                                                           8(a)                                                                              1:1.54 1000°C (3 hr)                                                                   1.5      0.235                                    30°C                                                                           8(b)                                                                              1:1.54 1000°C (3 hr)                                                                   1.5      0.232                                    40°C                                                                           8(c)                                                                              1:1.54 1000°C (3 hr)                                                                   1.5      0.215                                    45°C                                                                           8(d)                                                                              1:1.54 1000°C (3 hr)                                                                   1.5      0.198                                    70°C                                                                           8(e)                                                                              1:1.54 1000°C (3 hr)                                                                   1.5      0.115                                    90°C                                                                           8(f)                                                                              1:1.54 1000°C (3 hr)                                                                   1.5      0.020                                    __________________________________________________________________________

As can be seen, there is a dramatic decrease in electrochemical activityas the drying temperature is increased over 70°C. It is believed that,even though the Ni₃ O₄ . 2H₂ O does not convert to a cubic inactive formuntil about 130°C; interlaminar bonding in the battery material startsto occur above about 60°C-70°C; and that this along with interlaminarwater loss apparently makes the material dried over about 65°Cineffective as a battery material.

What exactly happens is not completely understood at this time; it isknown, however, that when the electrode material, containing a mixtureof Ni (II) and Ni (III) forms, and which is believed to have astoichiometry of nickel hydrated oxide Ni₃ O₄ . 2H₂ O, is dried attemperatures over 65°C, it becomes progressively inactive and is notuseful as a battery material. It is critical then that the activematerial of this invention only be dried between about 15°C-65°C andpreferably at 25°C. At the higher temperatures of about 65°C a highhumidity environment could be used to minimize interlaminar water loss.

The infrared and Raman spectra of the fully hydrolyzed product,containing nickel forms which correspond to a stoichiometry of Ni₃ O₄ .2H₂ O, when dried below 65°C, shows a center of symmetry and layer-like--O-Ni-ONi-O-Ni-O-- structure with water molecules dispersed ininterlaminar positions. The crystalline layer structure is hexagonal andbelongs to the same group D_(d3) ³ (P3m).

Unexpectedly, only the Na₂ O₂ + NiO reaction product, when thecomponents are reacted within critical weight percent, temperature andtime ranges, and when combined with critical weight percent Co duringfusion, during hydrolysis, after hydrolysis or after plaque pasting,provides suitable high performance active battery material for use inmaking battery electrode plates. This active material, formed byhydrolyzing NaNiO₂ and adding about 2-12 wt% Co based on NiO plus Co hasa capacity of at least 0.185 amp-hr/gram. When it is used in a metallicplaque it provides an electrode that can be alternately stacked in acontainer opposite negative electrodes, such as for example electrodescontaining iron active battery material, with separators therebetweenand a suitable caustic electrolyte contacting the electrodes andseparators, with suitable electrical connections, to provide a battery.

We claim as our invention:
 1. A method of producing a battery electrode plate containing active battery material, comprising the steps of:a. mixing NiO with Na₂ O₂ in a weight ratio of NiO:Na₂ O₂ of between about 1:1.35 to about 1:2.1; b. heating the mixture of NiO and Na₂ O₂ between about 800°C - 1150°C, for about 1/2 hour to 8 hours, to melt the mixture and to form NaNiO₂ ; c. hydrolyzing the NaNiO₂ in water at between about 20°C-95°C to form active battery material and then washing the active battery material; d. maintaining the activity of the battery material by maintaining the temperature of the material below about 65°C; and e. applying the battery material to a porous metallic plaque.
 2. The method of claim 1 wherein the NiO is substantially pure, and cobalt, selected from the group consisting of Co, Co₂ O₃, Co₃ O₄ and CoO and their mixtures is added to the materials.
 3. The method of claim 1 wherein the NiO is substantially pure, and cobalt additive as substantially pure cobalt hydroxide is added after hydrolysis of the reaction product comprising NaNiO₂.
 4. The method of claim 1 wherein cobalt additive as a water soluble cobalt salt is added during hydrolysis of the reaction product comprising NaNiO₂.
 5. The method of claim 1 wherein cobalt additive as a water soluble cobalt salt is added after hydrolysis
 6. The method of claim 1 wherein cobalt additive as a water soluble cobalt salt is added after applying the battery material to the porous metallic plaque.
 7. A method of producing a battery electrode plate containing active battery material, comprising the steps of:a. mixing an admixture of NiO and cobalt material selected from the group consisting of Co, Co₂ O₃, Co₃ O₄ and CoO and their mixtures and Na₂ O₂, wherein the weight ratio of NiO:Na₂ O₂ is between about 1:1.35 to about 1:2.1 and the amount of Co in the cobalt material is between about 2-12 wt% based on NiO plus Co content; b. heating the admixture at a temperature between about 800°C-1150°C for about one-half hour to 8 hours to melt the admixture and provide a reaction product consisting essentially of NaNiO₂ and then cooling the reaction product NaCoO₂ ; c. hydrolyzing the reaction product, forming an active battery material; d. washing the active battery material; and then maintaining the activity of the battery material by maintaining the temperature of the material below about 65°C.; and e. applying the active battery material to a metallic plaque to provide a battery electrode.
 8. The method of claim 7, wherein the reaction product is hydrolyzed in water, the active battery material contains from about 0.5 to 5 wt% unreacted NaNiO₂ and NaCoO₂ and the active battery material is dried between about 15°C-65°C after step (d).
 9. The method of claim 7, wherein the admixture in step (b) is heated at a temperature of between about 850°C-1100°C for between about 1/2-8 hours to melt the mixture and the NiO, cobalt material and Na₂ O₂ are in substantially pure form.
 10. The method of claim 8, wherein the reaction product is cooled to a temperature below about 95°C before step (c), and the water used in the hydrolysis step has a temperature of between about 20°C-95°C.
 11. The method of claim 8, wherein the reaction product is cooled to between about 20°-95°C before step (c), the water used in the hydrolysis step has a temperature of between about 20°C-35°C, the active battery material comprises Ni hydroxide forms which have a weight ratio of Ni (II) hydroxide:Ni (III) hydroxide of over 1:2 and the active battery material is washed after hydrolysis until neutral to litmus.
 12. The method of claim 11, wherein the reaction product is hydrolyzed by immersion in water and the NiO, cobalt material and Na₂ O₂ contain no more than 5% impurities selected from the group consisting of mercury, silver, cadmium, lead, magnesium, chromium, calcium, zirconium and barium compounds.
 13. The method of claim 11, wherein the amount of Co in the mixture of step (a) is between about 4-8 wt%, the active battery material Ni hydroxide forms comprise a material having a stoichimetry of Ni₃ O₄ . 2H₂ O, and the active battery material is applied to the metallic plaque in aqueous slurry form.
 14. The method of claim 13, wherein the metallic plaque is between 90-95% porous and comprises relatively smooth contacting metal fibers.
 15. The method of claim 14, wherein the metal fibers are diffusion bonded before coating, wherein there is only an interdiffusion of atoms across the fibers interface. 